'\" t
.TH "SD_ID128_TO_STRING" "3" "" "systemd 257" "sd_id128_to_string"
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.SH "NAME"
sd_id128_to_string, SD_ID128_TO_STRING, SD_ID128_STRING_MAX, sd_id128_to_uuid_string, SD_ID128_TO_UUID_STRING, SD_ID128_UUID_STRING_MAX, sd_id128_from_string \- Format or parse 128\-bit IDs as strings
.SH "SYNOPSIS"
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.nf
#include <systemd/sd\-id128\&.h>
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#define SD_ID128_STRING_MAX 33U
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#define SD_ID128_UUID_STRING_MAX 37U
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#define SD_ID128_TO_STRING(id) \&...
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#define SD_ID128_TO_UUID_STRING(id) \&...
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.HP \w'char\ *sd_id128_to_string('u
.BI "char *sd_id128_to_string(sd_id128_t\ " "id" ",\ char\ " "s" "[static\ SD_ID128_STRING_MAX]);"
.HP \w'char\ *sd_id128_uuid_string('u
.BI "char *sd_id128_uuid_string(sd_id128_t\ " "id" ",\ char\ " "s" "[static\ SD_ID128_UUID_STRING_MAX]);"
.HP \w'int\ sd_id128_from_string('u
.BI "int sd_id128_from_string(const\ char\ *" "s" ",\ sd_id128_t\ *" "ret" ");"
.SH "DESCRIPTION"
.PP
\fBsd_id128_to_string()\fR
formats a 128\-bit ID as a character string\&. It expects the ID and a string array capable of storing 33 characters (\fBSD_ID128_STRING_MAX\fR)\&. The ID will be formatted as 32 lowercase hexadecimal digits and be terminated by a
\fBNUL\fR
byte\&.
.PP
\fBSD_ID128_TO_STRING()\fR
is a macro that wraps
\fBsd_id128_to_string()\fR
and passes an appropriately sized buffer as second argument, allocated as C99 compound literal\&. Each use will thus implicitly acquire a suitable buffer on the stack which remains valid until the end of the current code block\&. This is usually the simplest way to acquire a string representation of a 128\-bit ID in a buffer that is valid in the current code block\&.
.PP
\fBsd_id128_to_uuid_string()\fR
and
\fBSD_ID128_TO_UUID_STRING()\fR
are similar to these two functions/macros, but format the 128\-bit values as RFC4122 UUIDs, i\&.e\&. a series of 36 lowercase hexadeciaml digits and dashes, terminated by a
\fBNUL\fR
byte\&.
.PP
\fBsd_id128_from_string()\fR
implements the reverse operation: it takes a 33 character string with 32 hexadecimal digits (either lowercase or uppercase, terminated by
\fBNUL\fR) and parses them back into a 128\-bit ID returned in
\fIret\fR\&. Alternatively, this call can also parse a 37\-character string with a 128\-bit ID formatted as RFC UUID\&. If
\fIret\fR
is passed as
\fBNULL\fR
the function will validate the passed ID string, but not actually return it in parsed form\&.
.PP
Note that when formatting and parsing 36 character UUIDs this is done strictly in Big Endian byte order, i\&.e\&. according to
\m[blue]\fBRFC4122\fR\m[]\&\s-2\u[1]\d\s+2
Variant 1 rules, even if the UUID encodes a different variant\&. This matches behaviour in various other Linux userspace tools\&. It\*(Aqs probably wise to avoid UUIDs of other variant types\&.
.PP
For more information about the
"sd_id128_t"
type see
\fBsd-id128\fR(3)\&. Note that these calls operate the same way on all architectures, i\&.e\&. the results do not depend on endianness\&.
.PP
When formatting a 128\-bit ID into a string, it is often easier to use a format string for
\fBprintf\fR(3)\&. This is easily done using the
\fBSD_ID128_FORMAT_STR\fR
and
\fBSD_ID128_FORMAT_VAL()\fR
macros\&. For more information see
\fBsd-id128\fR(3)\&.
.SH "RETURN VALUE"
.PP
\fBsd_id128_to_string()\fR
always succeeds and returns a pointer to the string array passed in\&.
\fBsd_id128_from_string()\fR
returns 0 on success, in which case
\fIret\fR
is filled in, or a negative errno\-style error code\&.
.SH "NOTES"
.PP
Functions described here are available as a shared library, which can be compiled against and linked to with the
\fBlibsystemd\fR\ \&\fBpkg-config\fR(1)
file\&.
.PP
The code described here uses
\fBgetenv\fR(3), which is declared to be not multi\-thread\-safe\&. This means that the code calling the functions described here must not call
\fBsetenv\fR(3)
from a parallel thread\&. It is recommended to only do calls to
\fBsetenv()\fR
from an early phase of the program when no other threads have been started\&.
.SH "HISTORY"
.PP
\fBsd_id128_to_string()\fR
and
\fBsd_id128_from_string()\fR
were added in version 187\&.
.PP
\fBsd_id128_uuid_string()\fR
was added in version 251\&.
.SH "SEE ALSO"
.PP
\fBsystemd\fR(1), \fBsd-id128\fR(3), \fBprintf\fR(3)
.SH "NOTES"
.IP " 1." 4
RFC4122
.RS 4
\%https://tools.ietf.org/html/rfc4122
.RE
